Substations in high and medium-voltage power networks include primary devices such as electrical cables, lines, bus bars, switches, power transformers and instrument transformers, which are generally arranged in switch yards and/or bays. These primary devices are operated in an automated way via a Substation Automation (SA) system. The SA system includes microprocessor based, programmable secondary devices, so-called Intelligent Electronic Devices (IEDs), which are responsible for protection, control and monitoring of the primary devices. The IEDs are generally assigned to one of three hierarchical levels, i.e. the station level, the bay or application level, and the process level, which is separated from the bay level by a process interface. The station level of the SA system includes an Operator Work Station (OWS) with a Human-Machine Interface (HMI) and a gateway to a Network Control Centre (NCC). IEDs on the bay level, also termed bay units, in turn are connected to each other and to the IEDs on the station level via an optical inter-bay or station bus.
IEDs on the process-level include, for example, sensors for voltage, current and gas density measurements, contact probes for sensing switch and transformer tap changer positions, or intelligent actuators for controlling a switchgear like circuit breakers or disconnectors. Breaker-IEDs, if shielded against electromagnetic disturbances, may even be directly integrated into the switchgear or respective intelligent primary equipment. Such process-level IEDs are connected to the bay units via a process bus, such as an optical bus, which can be considered as the process interface replacing the hard-wired process-interface that conventionally connects the switchyard to the bay level equipment.
A communication standard for communication between the secondary devices of a substation has been introduced by the International Electrotechnical Committee (IEC) as part of the standard IEC 61850 entitled “Communication Networks and Systems in Substations”. For non-time critical messages, the communication between the IEDs is handled via a Manufacturing Message Specification (MMS) application level protocol and a reduced Open Systems Interconnection (OSI) protocol stack with the Transmission Control Protocol (TCP) and Internet Protocol (IP) in the transport and network layer, respectively, and Ethernet and/or RS-232C as physical media. On the other hand, time critical messages, which are referred to as Generic Object Oriented Substation Events (GOOSE, part IEC 61850-8-1 of the standard), build directly on the Ethernet link layer of the communication stack. Very time-critical signals at the process level, such as trip commands and measured analog voltages or currents, use a simplified variant of GOOSE known as Sampled Values (SV, part IEC 61850-9-2 of the standard) that also builds directly on the Ethernet link layer.
A Voltage Transformer (VT) can be connected to a bus bar of a substation and provide instantaneous values of the bus bar voltage, which are then distributed to all the bays connected to the bus bar. However, VTs are costly, and if they have to be maintained or replaced, the concerned part of the switch yard can be put out of operation for a long time. In addition, a VT used for a synchrocheck function at a bus bar represents a single point of failure, because a bay can cannot be connected to the bus bar if this bus VT fails.
To avoid making a bus bar VT a single point of failure, a bay VT of a bay currently connected to the bus bar is selected and switched to a dedicated cable or voltage bus of an SA system. The central cable then distributes this bay voltage as the bus bar voltage to all the bays needing it. Such SA systems use a relay or wiring logic to determine which bay VT is connected to the bus bar. However, this solution of bus bar voltage determination can result in electrical problems as well as other complications, if the logic does not work correctly or there are time delays. For example, if two VTs are accidentally connected to the central cable at the same time, it would destroy them both, causing possible danger to human beings near them. Also, in this case, the common relay or wiring logic becomes the single point of failure.
The advent of digital controllers rectified the issues associated with the accuracy of the logic and enhanced its reliability by expressing the selection logic as Boolean formulas calculated in the controllers. However, all the VTs have to be switched with some relay contacts to the bus bar related cable. Therefore, the danger of connecting them erroneously still persists, especially if the de-centrally implemented selection logic is too simple. A solution to this problem is centrally implemented selection logic on a controller IED, which assures that only one VT is switched to the voltage bus cable. However, like the VT and the relay logic, the centrally implemented selection logic also becomes a single point of failure. In addition, these solutions also need a dedicated cable for the voltage running through all the bays, and A/D (analog-to-digital) converters at all the bays needing the bus bar voltage.